Power output in hydraulic systems

ABSTRACT

The power output of a hydraulic system is improved by operating the hydraulic system with a hydraulic fluid having a VI of at least 130.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the increase of power output in hydraulic pumps and motors, achieved by the use of hydraulic fluids with high viscosity index. Use of such fluids can increase the power output of the system without any modification of the hardware.

2. Description of the Related Art

Hydraulic systems are designed to transmit energy and apply large forces with a high degree of flexibility and control. It is desirable to build systems that efficiently convert input energy from an engine, electric motor, or other source into usable work. Hydraulic power can be used to create rotary or linear motion, or to store energy for future use in an accumulator. Hydraulic systems provide a significantly more accurate and adjustable means to transmit energy than electrical or mechanical systems. In general, hydraulic systems are reliable, efficient, and cost effective, leading to their wide use in the industrial world. The fluid power industry is constantly improving the cost effectiveness of hydraulic systems by employing new mechanical components and materials of construction.

Water and many other liquids can be utilized to make practical use of Pascal's Law, which states that a fluid compressed in a closed container will transmit the resulting pressure throughout the system undiminished and equal in all directions.

Standard “HM” monograde oil is typically selected as it is the lowest cost option and has a long history of dependable performance with no maintenance issues. Outdoor applications of fluid power that experience wide variations in temperature will make use of lower viscosity grade fluids in the winter and higher viscosity grade fluids in the summer. Some hydraulic fluids are formulated with PAMA additives as viscosity index improvers, in order to achieve good low temperature fluidity properties under cold start-up conditions (“HV” grade oils). PAMA additives are not known to offer any other performance benefits.

E.g., the document WO 2005108531 describes the use of hydraulic fluids comprising PAMA additives in order to reduce the temperature increase of a hydraulic fluid under work load. However, an improvement with regard to power output is not indicated or suggested by that document.

Additionally, the document WO 2005014762 discloses a functional fluid having an improved fire resistance. The fluid can be used in hydraulic systems. However, the document is silent with regard to the power output of the system using such a fluid.

Achieving higher power output in a hydraulic system is typically achieved by selecting a larger pump, or by other hardware construction improvements of the unit providing mechanical energy to the hydraulic system. However, such an approach is usually connected with higher energy consumption and increased cost.

A further common object of the background art is the improvement of the volume output. According to background art, these objects are conventionally achieved by a combustion engine or an electric motor having more power. However, such approach is usually connected with higher energy consumption and increased cost, and is often constrained by space or weight limitations.

SUMMARY OF THE INVENTION

Taking into consideration the background art, it is an object of the present invention to provide hydraulic systems having increased power output in order to facilitate increased work loads and improved productivity. Increased power output can be used to generate increased digging force, lifting capacity, or machine speed. Furthermore, it is an object of the present invention to improve the lifetime and the service interval of the hydraulic system.

These and other objects have been achieved by the present invention the first embodiment of which includes a method of improving a power output of a hydraulic system, comprising:

operating said hydraulic system with a hydraulic fluid having a VI of at least 130.

In another embodiment, the present invention relates to a method of improving a volume output of a hydraulic system, comprising:

operating said hydraulic system with a hydraulic fluid having a VI of at least 130;

wherein the volume output of said hydraulic system is increased compared to the volume output of a system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor.

In another embodiment, the present invention provides a hydraulic fluid, having a VI of at least 130.

In yet another embodiment, the present invention provides that the above hydraulic fluid improves a power output of a hydraulic system compared to the power output of a hydraulic system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor.

Further, the present invention provides a hydraulic system, comprising:

the above hydraulic fluid,

wherein said hydraulic system is a military hydraulic system, a hydraulic launch assist system for hydraulic hybrid vehicle propulsion, an industrial hydraulic system, marine hydraulic system, mining hydraulic system, mobile equipment hydraulic system or combinations thereof.

Further, the present invention provides a hydraulic system, comprising:

the above hydraulic fluid,

wherein said hydraulic system comprises at least one unit providing mechanical energy, at least one unit that converts mechanical energy into hydraulic power, at least one pipe for transmitting hydraulic fluid under pressure and at least a unit that converts the hydraulic power of the hydraulic fluid into mechanical work.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a comparison of power output at 5000 psi in a piston pump (data from Table 6)

FIG. 2 shows a comparison of power output at 3000 psi in a vane pump (data from Table 7)

DETAILED DESCRIPTION OF THE INVENTION

Especially, the improvement of hydraulic power output is achieved by the use of a fluid according to the present invention.

The use of a fluid having a VI of at least 130 provides an unexpected increase in the hydraulic power output of a pump. The increased power output from the pump results in increased power output from the hydraulic motor (cylinder or rotary motor).

The hydraulic fluid of the present invention shows an improved low temperature performance and broader temperature operating window. Furthermore, the hydraulic fluid provides an improvement in volume output. Additionally, a hydraulic system using a hydraulic fluid having a VI of at least 130 shows an improvement of the power drop, especially at a high load of the unit providing mechanical work. Therefore, the constancy of the power output is improved by the use of the present invention.

The hydraulic fluid of the present invention can be sold on a cost favorable basis with fast investment payback time.

It is also possible to design a hydraulic system utilizing a high viscosity index hydraulic fluid that operates at a lower pressure level, and generates an equivalent amount of hydraulic power output with lower pump input energy. A system that operates at lower pressure will have longer component life (seals, hoses, wear surfaces, fluid), and will result in lower fluid operating temperature.

The hydraulic fluid of the present invention exhibits good resistance to oxidation and is chemically very stable, compared to a standard HM fluid. Specifically, the monograde hydraulic fluid of the present invention is a HM fluid that exhibits good resistance to oxidation and is chemically very stable, compared to a standard HM fluid. In the context of the present invention, “HM” is a designation for monograde hydraulic fluid based on mineral oil, containing rust and oxidation inhibitors with antiwear characteristics. “HV” is a designation for an HM fluid with improved viscosity/temperature properties, intended for operation over a wide range of ambient temperatures. Both designations are defined by the ASTM D 6158 specification.

The viscosity of the hydraulic fluid of the present invention can be adjusted over a broad range.

Furthermore, the hydraulic fluids of the present invention are appropriate for high pressure applications, in the range of 100 to 700 bars. The hydraulic fluids of the present invention show a minimal change in viscosity in-service due to good shear stability.

Additionally, the improvement of power output and system productivity can be achieved without constructional changes of the hydraulic system. Consequently, the power output of both new and old hydraulic systems can be improved at very low cost. The composition of such hydraulic fluids are fully compatible with existing elastomeric materials used in seals, bladders, and hoses making them immediately acceptable for use in existing industrial hydraulic systems.

The hydraulic fluid used according to the present invention has a viscosity index of at least 130, preferably at least 150, more preferably at least 180 and most preferably at least 200. According to a preferred embodiment of the present invention, the viscosity index is in the range of 150 to 400, more preferably 200 to 300. The viscosity index (VI) can be determined according to ASTM D 2270. Viscosity index (VI) as defined by ASTM D2270 is the relationship between the kinematic viscosity at 40° C. and the kinematic viscosity at 100° C.

The use according to the present invention provides an improvement of the power output of a hydraulic system. The expression “power output” means energy usable as work, typically measured and quantified as output torque from a rotary hydraulic motor in horsepower or kilowatts.

Preferably, the fluid of the present invention is effective in increasing the power output of the hydraulic system by at least 3%, more preferably at least 5% and more preferably at least 10%, compared to the power output of a system using a monograde hydraulic fluid having a VI of about 100 operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor. Therefore, equal amounts of energy are consumed (fuel or electricity), however, the system using the high VI fluid will produce more usable output power in an equal period of time.

According to a preferred embodiment of the present invention, the volume output is increased. Preferably, the fluid of the present invention is effective in increasing the volume output of the hydraulic system by at least 3%, more preferably at least 5% and more preferably at least 10%, compared to the volume output of a system using a monograde hydraulic fluid having a VI of about 100 operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor. The expression “volume output” means volume provided to a hydraulic motor usable as work at a specific pressure difference, typically measured and quantified in m³ or liter.

The present invention could additionally provide a method for improving the constancy of the power output. Surprisingly, the constancy of the power output can also be increased at the maximum load. For example, the drop of the power output after at least 10 minutes of operating time is preferably at most 3%, measured at a load of 90% of the maximum load or more of a unit providing mechanical energy.

Preferably, the engine speed of a unit providing mechanical energy is in the range of 1000 to 3000 rpm, more preferably in the range of 1400 to 2000 rpm.

The improvements mentioned above can be used to increase the performance of a hydraulic system in an astonishing manner. By providing a system having a low and postponed drop of the power output, the system can be used at the power limits of the unit creating mechanical energy. Therefore, a defined work can be done within a shorter time without the need of constructional changes of the system. Preferably, the engine speed of a unit providing mechanical energy is maintained at a constant rate and the system delivers an increased level of hydraulic power.

According to a preferred embodiment of the present invention, the hydraulic system can be designed to operate at a lower pressure, such that the output power is equivalent to that delivered by a reference system using a hydraulic fluid with a VI of 100. E.g., in an excavator the shovel can be changed. By using a lower pressure, the lifetime and the service intervals of the hydraulic system can be improved in an astonishing manner.

According to a preferred embodiment of the present invention, the hydraulic system can demonstrate an improvement in the ratio of hydraulic power output to power input, such that the ratio of power output/power input is preferably improved by at least 3%, more preferably at least 5% compared to that delivered by a reference system using a hydraulic fluid with a VI of 100.

In a preferred embodiment, the hydraulic fluid according to the present invention increases a constancy of the power output, preferably increases a constancy of the power output at the maximum load. Preferably, the drop of the power output after at least 10 minutes of operating time is at most 3%, measured at a load of 90% of the maximum load or more of a unit providing mechanical energy. In the hydraulic system according to the present invention using the hydraulic fluid, the engine speed of a unit providing mechanical energy is maintained at a constant rate and the system delivers an increased level of hydraulic power. Preferably, the engine speed of a unit providing mechanical energy is in the range of 1000 to 3000 rpm. More preferably, the engine speed of a unit providing mechanical energy is in the range of 1400 to 2000 rpm. The pressure provided by a unit providing hydraulic power is in the range of 50 to 700 bar, preferably, in the range of 150 to 350 bar. Preferably, the hydraulic system is designed to operate at a lower pressure, such that the output power is equivalent to that delivered by a reference system using a hydraulic fluid with a VI of 100. Preferably, the hydraulic system demonstrates an improvement in the ratio of hydraulic power output to power input, such that the ratio of power output/power input is improved by at least 3%, compared to that delivered by a reference system using a hydraulic fluid with a VI of 100.

The hydraulic fluid according to the present invention may be obtained by mixing a base fluid and a polymeric viscosity index improver. The hydraulic fluid comprises at least 60% by weight of at least one base fluid. Preferably, the hydraulic fluid comprises at least 60% by weight of at least one base hydraulic fluid having a viscosity index of 120 or less. Further, the hydraulic fluid may comprise a member selected from the group consisting of a mineral oil, a synthetic oil and mixtures thereof.

The hydraulic fluid may comprises an API group I oil, API group II oil, API group III oil, a API group IV oil, API group V oil, a Fischer-Tropsch (GTL) derived oil or mixtures thereof. In addition, the hydraulic fluid may comprises a polyalphaolefin, a carboxylic ester, a vegetable ester, a phosphate ester, a polyalkylene glycol or mixtures thereof.

In another embodiment, the hydraulic fluid may comprises at least one polymer. The polymer comprises polymerized units from monomers selected from the group consisting of acrylate monomers, methacrylate monomers, fumarate monomers, maleate monomers and mixtures thereof. Preferably, the hydraulic fluid comprises a polyalkylmethacrylate polymer.

In a preferred embodiment, the polymer is obtained by polymerizing a mixture of olefinically unsaturated monomers, which comprises

a) 0-100 wt % of one or more ethylenically unsaturated ester compounds of formula (I) based on the total weight of the ethylenically unsaturated monomers:

wherein

R is hydrogen or methyl,

R¹ is a linear or branched alkyl residue with 1-6 carbon atoms, R² and R³ each independently represent hydrogen or a group of the formula —COOR′, wherein R′ is hydrogen or an alkyl group with 1-6 carbon atoms,

b) 0-100 wt % of one or more ethylenically unsaturated ester compounds of formula (II) based on the total weight of the ethylenically unsaturated monomers:

wherein

R is hydrogen or methyl,

R⁴ is a linear or branched alkyl residue with 7-40 carbon atoms,

R⁵ and R⁶ independently are hydrogen or a group of the formula —COOR″, wherein R″ is hydrogen or an alkyl group with 7-40 carbon atoms,

c) 0-50 wt % of comonomers based on the total weight of the ethylenically unsaturated monomers.

The polymer may be obtained by a polymerization in a API group II mineral oil or API group III mineral oil. In addition, the polymer may be obtained by a polymerization in a polyalphaolefin. In another embodiment, the polymer may be obtained by polymerizing a dispersant monomer. The polymer may be obtained by polymerizing a vinyl monomer containing an aromatic group.

Preferably, the polymer has a weight average molecular weight in the range of 10000 to 200000 g/mol.

The hydraulic fluid may comprises 0.5 to 40% by weight of a polymer, preferably, 3 to 20% by weight of a polymer.

The hydraulic fluid may comprises at least two polymers having a different monomer composition. At least one of the polymers may a polyolefin. In another embodiment, at least one of the polymers comprises units derived from at least one alkyl ester monomer.

A weight ratio of the polyolefin and the polymer comprising units derived from at least one alkyl ester monomer is in the range of 1:10 to 10:1.

Preferably, the hydraulic fluid may comprise an oxygen containing compound selected from the group consisting of carboxylic acid esters, polyether polyols, organophosphorus compounds and mixtures thereof.

The oxygen containing compound is preferably a carboxylic ester containing at least two ester groups. The oxygen containing compound may be a diester of a carboxylic acid containing 4 to 12 carbon atoms. Preferably, the oxygen containing compound is an ester of a polyol. The ISO viscosity grade of the hydraulic fluid may be in the range of 15 to 150.

The hydraulic fluid may be used at a temperature in the range of −40° C. to 120° C.

Preferably, the hydraulic fluid comprises a member selected from the group consisting of antioxidants, antiwear agents, corrosion inhibitors, defoamers and mixtures thereof.

The viscosity of the hydraulic fluid of the present invention can be adapted with in wide range, according to the requirements of the hydraulic pump/motor manufacturer. ISO VG 15, 22, 32, 46, 68, 100, 150 fluid grades can be achieved, e.g.

ISO 3448 Maximum Viscosity Typical Viscosity, Minimum Viscosity, Viscosity, cSt @ Grades cSt @ 40° C. cSt @ 40° C. 40° C. ISO VG 15 15.0 13.5 16.5 ISO VG 22 22.0 19.8 24.2 ISO VG 32 32.0 28.8 35.2 ISO VG 46 46.0 41.4 50.6 ISO VG 68 68.0 61.2 74.8 ISO VG 100 100.0 90.0 110.0 ISO VG 150 150.0 135.0 165.0

Preferably the kinematic viscosity at 40° C. according to ASTM D 445 of is the range of 15 mm²/s to 150 mm²/s, preferably 28 mm²/s to 110 mm²/s. The kinematic viscosity at 40° C. includes all values and subvalues therebetween, especially including 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130 and 140 mm²/s.

For the use according to the present invention, preferred hydraulic fluids are NFPA (National Fluid Power Association) multigrade fluids, e.g. double, triple, quadra and/or penta grade fluids as defined by NFPA T2.13.13-2002.

Preferred fluids comprise at least a mineral oil and/or a synthetic oil.

Mineral oils are substantially known and commercially available. They are in general obtained from petroleum or crude oil by distillation and/or refining and optionally additional purification and processing methods, especially the higher-boiling fractions of crude oil or petroleum fall under the concept of mineral oil. In general, the boiling point of the mineral oil is higher than 200° C., preferably higher than 300° C., at 5000 Pa. Preparation by low temperature distillation of shale oil, coking of hard coal, distillation of lignite under exclusion of air as well as hydrogenation of hard coal or lignite is likewise possible. To a small extent mineral oils are also produced from raw materials of plant origin (for example jojoba, rapeseed (canola), sunflower, and soybean oil) or animal origin (for example tallow or neat foot oil). Accordingly, mineral oils exhibit different amounts of aromatic, cyclic, branched and linear hydrocarbons, in each case according to origin.

In general, one distinguishes paraffin-base, naphthenic and aromatic fractions in crude oil or mineral oil, where the term paraffin-base fraction stands for longer-chain or highly branched isoalkanes and naphthenic fraction stands for cycloalkanes. Moreover, mineral oils, in each case according to origin and processing, exhibit different fractions of n-alkanes, isoalkanes with a low degree of branching, so called monomethyl-branched paraffins, and compounds with heteroatoms, especially O, N and/or S, to which polar properties are attributed. However, attribution is difficult, since individual alkane molecules can have both long-chain branched and cycloalkane residues and aromatic components. For purposes of this present invention, classification can be done in accordance with DIN 51 378. Polar components can also be determined in accordance with ASTM D 2007.

The fraction of n-alkanes in the preferred mineral oils is less than 3 wt %, and the fraction of O, N and/or S-containing compounds is less than 6 wt %. The fraction of aromatic compounds and monomethyl-branched paraffins is in general in each case in the range of 0-40 wt %. In accordance with one interesting aspect, mineral oil comprises mainly naphthenic and paraffin-base alkanes, which in general have more than 13, preferably more than 18 and especially preferably more than 20 carbon atoms. The fraction of these compounds is in general at least 60 wt %, preferably at least 80 wt %, without any limitation intended by this. A preferred mineral oil contains 0.5-30 wt % aromatic components, 15-40 wt % naphthenic components, 35-80 wt % paraffin-base components, up to 3 wt % n-alkanes and 0.05-5 wt % polar components, in each case with respect to the total weight of the mineral oil.

An analysis of especially preferred mineral oils, which was done with traditional methods such as urea dewaxing and liquid chromatography on silica gel, shows, for example, the following components, where the percentages refer to the total weight of the relevant mineral oil:

n-alkanes with about 18-31 C atoms: 0.7-1.0%,

low-branched alkanes with 18-31 C atoms: 1.0-8.0%,

aromatic compounds with 14-32 C atoms: 0.4-10.7%,

iso- and cycloalkanes with 20-32 C atoms: 60.7-82.4%,

polar compounds: 0.1-0.8%,

loss: 6.9-19.4%.

Valuable advice regarding the analysis of mineral oil as well as a list of mineral oils that have other compositions can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition on CDROM, 1997, under the entry “lubricants and related products.”

Preferably, the hydraulic fluid is based on mineral oil from API Group I, II, or III. According to a preferred embodiment of the present invention, a mineral oil containing at least 90% by weight saturates and at most about 0.03% sulfur measured by elemental analysis is used. Especially, API Group II oils are preferred.

API Group IV and V synthetic oils are, among other substances, organic esters like carboxylic esters and phosphate esters; organic ethers like silicone oils and polyalkylene glycol; and synthetic hydrocarbons, especially polyolefins and Fischer-Tropsch (GTL) derived base oils. They are for the most part somewhat more expensive than the mineral oils, but they have advantages with regard to performance. For an explanation reference is made to the 5 API classes of base oil types (API: American Petroleum Institute).

American Petroleum Institute (API) Base Oil Classifications Sulfur Saturates Base stock Group Viscosity Index (weight %) (weight %) Group I 80-120 >0.03 <90 Group II 80-120 <0.03 >90 Group III >120 <0.03 >90 Group IV all synthetic >120 <0.03 >99 Polyalphaolefins (PAO) Group V all not >120 <0.03 included in Groups I-IV, e.g. esters, polyalkylene glycols

The Fischer-Tropsch derived base oil may be any Fischer-Tropsch derived base oil as disclosed in for example EP-A-776959, EP-A-668342, WO-A-9721788, WO-0015736 WO-0014188, WO-0014187, WO-0014183, WO-0014179, WO-0008115, WO-9941332, EP-1029029, WO-0118156 and WO-0157166. A thorough discussion of GTL technology can be found in: Henderson, H. E., “Gas to Liquids”, Chapter 19 of Synthetics, Mineral Oils, and Bio-Based Lubricants—Chemistry and Technology. Rudnick, L. R., (editor), CRC Press, Taylor and Francis, 2006, p. 317.

Synthetic hydrocarbons, include especially polyolefins. Especially polyalphaolefins (PAO) are preferred. These compounds are obtainable by polymerization of alkenes, especially alkenes having 3 to 12 carbon atoms, like propene, hexene-1, octene-1, and dodecene-1. Preferred PAOs have a number average molecular weight in the range of 200 to 10000 g/mol, more preferably 500 to 5000 g/mol.

According to a preferred aspect of the present invention, the hydraulic fluid may comprise an oxygen containing compound selected from the group of carboxylic acid esters, polyether polyols and/or organophosphorus compounds. Preferably, the oxygen containing compound is a carboxylic ester containing at least two ester groups, a diester of carboxylic acids containing 4 to 12 carbon atoms and/or a ester of a polyol. By using an oxygen containing compound as a basestock, the fire resistance of the hydraulic fluid can be improved.

Phosphorus ester fluids can be used as a component of the hydraulic fluid such as alkyl aryl phosphate ester; trialkyl phosphates such as tributyl phosphate or tri-2-ethylhexyl phosphate; triaryl phosphates such as mixed isopropylphenyl phosphates, mixed t-butylphenyl phosphates, trixylenyl phosphate, or tricresylphosphate. Additional classes of organophosphorus compounds are phosphonates and phosphinates, which may contain alkyl and/or aryl substituents. Dialkyl phosphonates such as di-2-ethylhexylphosphonate; alkyl phosphinates such as di-2-ethylhexylphosphinate are useful. As the alkyl group herein, linear or branched chain alkyls comprising 1 to 10 carbon atoms are preferred. As the aryl group herein, aryls comprising 6 to 10 carbon atoms that maybe substituted by alkyls are preferred. Especially, the hydraulic fluids may contain 0 to 60% by weight, preferably 5 to 50% by weight organophosphorus compounds. The amount of organophosphorous compounds includes all values and subvalues therebetween, especially including 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55% by weight.

As the carboxylic acid esters reaction products of alcohols such as polyhydric alcohol, monohydric alcohol and the like, and fatty acids such as mono carboxylic acid, polycarboxylic acid and the like can be used. Such carboxylic acid esters can of course be a partial ester.

Carboxylic acid esters may have one carboxylic ester group having the formula R—COO—R, wherein R is independently a group comprising 1 to 40 carbon atoms. Preferred ester compounds comprise at least two ester groups. These compounds may be based on polycarboxylic acids having at least two acidic groups and/or polyols having at least two hydroxyl groups.

The polycarboxylic acid residue usually has 2 to 40, preferably 4 to 24, especially 4 to 12 carbon atoms. Useful polycarboxylic acids esters are, e.g., esters of adipic, azelaic, sebacic, phthalate and/or dodecanoic acids. The alcohol component of the polycarboxylic acid compound preferably comprises 1 to 20, especially 2 to 10 carbon atoms.

Examples of useful alcohols are methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol and octanol. Furthermore, oxoalcohols can be used such as diethylene glycol, triethylene glycol, tetraethylene glycol up to decamethylene glycol.

Especially preferred compounds are esters of polycarboxylic acids with alcohols comprising one hydroxyl group. Examples of these compounds are described in Ullmanns Encyclopadie der Technischen Chemie, third edition, vol. 15, page 287-292, Urban & Schwarzenber (1964)).

Useful polyols to obtain ester compounds comprising at least two ester groups contain usually 2 to 40, preferably 4 to 22 carbon atoms. Examples are neopentyl glycol, diethylene glycol, dipropylene glycol, 2,2-dimethyl-3-hydroxypropyl-2′,2′-dimethyl-3′-hydroxy propionate, glycerol, trimethylolethane, trimethanol propane, trimethylolnonane, ditrimethylol-propane, pentaerythritol, sorbitol, mannitol and dipentaerythritol. The carboxylic acid component of the polyester may contain 1 to 40, preferably 2 to 24 carbon atoms. Examples are linear or branched saturated fatty acids such as formic acid, acetic acid, propionic acid, octanoic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecanoic acid, lauric acid, tridecanoic acid, myrisric acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearic acid, nonadecanoic acid, arachic acid, behenic acid, iso-myiristic acid, isopalmitic acid, isostearic acid, 2,2-dimethylbutanoic acid, 2,2-dimethylpentanoic acid, 2,2-dimethyloctanoic acid, 2-ethyl-2,3,3-trimethylbutanoic acid, 2,2,3,4-tetramethylpentanoic acid, 2,5,5-trimethyl-2-t-butylhexanoic acid, 2,3,3-trimethyl-2-ethylbutanoic acid, 2,3-dimethyl-2-isopropylbutanoic acid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic, acid; linear or branched unsaturated fatty such as linoleic acid, linolenic acid, 9 octadecenoic acid, undecenoic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, and commercial grades of oleic acid from a variety of animal fat or vegetable oil sources. Mixtures of fatty acids such as tall oil fatty acids can be used.

Especially useful compounds comprising at least two ester groups are, e.g., neopentyl glycol tallate, neopentyl glycol dioleate, propylene glycol tallate, propylene glycol dioleate, diethylene glycol tallate, and diethylene glycol dioleate.

Many of these compounds are commercially available from Inolex Chemical Co. under the trademark Lexolube 2G-214, from Cognis Corp. under the trademark ProEco 2965, from Uniqema Corp. under the trademarks Priolube 1430 and Priolube 1446 and from Georgia Pacific under the trademarks Xtolube 1301 and Xtolube 1320.

Furthermore, ethers are useful as a component of the hydraulic fluid. Preferably, polyether polyols are used as a component of the hydraulic fluid of the present invention. Examples are polyalkylene glycols like, e.g., polyethylene glycols, polypropylene glycols and polybutylene glycols. The polyalkylene glycols can be based on mixtures of alkylene oxides. These compounds preferably comprise 1 to 40 alkylene oxide units, more preferably 5 to 30 alkylene oxide units. Polybutylene glycols are preferred compounds for anhydrous fluids. The polyether polyols may comprise further groups, like e.g., alkylene or arylene groups comprising 1 to 40, especially 2 to 22 carbon atoms.

According to another aspect of the present invention, the hydraulic fluid is based on a synthetic basestock comprising polyalphaolefin (PAO), carboxylic esters (diester, or polyol ester), a vegetable ester, phosphate ester (trialkyl, triaryl, or alkyl aryl phosphates), and/or polyalkylene glycol (PAG). Preferred synthetic basestocks are API Group IV and/or Group V oils.

Preferably, the hydraulic fluid is obtainable by mixing at least two components. At least one of the components shall be a base oil. The expression base oil includes mineral oil and/or synthetic oil on which the hydraulic fluid could be based as mentioned above. Preferably, the hydraulic fluid comprises at least 60% by weight of base oil. Preferably, at least one of the components may have a viscosity index of 120 or less. According to a preferred embodiment, the hydraulic fluid may comprise at least 60% by weight of at least one component having a viscosity index of 120 or less.

Particularly, a polymeric viscosity index improver can be used as a component of the hydraulic fluid. Viscosity index improvers are e.g. disclosed in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition on CD-ROM, 1997.

Preferred polymers useful as VI improvers comprise units derived from alkyl esters having at least one ethylenically unsaturated group. Preferred polymers are obtainable by polymerizing, in particular, (meth)acrylates, maleates and fumarates. The term (meth)acrylates includes methacrylates and acrylates as well as mixtures of the two. The alkyl residue can be linear, cyclic or branched.

Mixtures to obtain preferred polymers comprising units derived from alkyl esters contain 0 to 100 wt %, preferably 0.5 to 90 wt %, especially 1 to 80 wt %, more preferably 1 to 30 wt %, more preferably 2 to 20 wt %, based on the total weight of the monomer mixture, of one or more ethylenically unsaturated ester compounds of formula (I)

where R is hydrogen or methyl, R¹ means a linear or branched alkyl residue with 1-6, especially 1 to 5 and preferably 1 to 3 carbon atoms, R² and R³ are independently hydrogen or a group of the formula —COOR′, where R′ means hydrogen or an alkyl group with 1-6 carbon atoms. The amount of a compound of formula (I) in the mixture includes all values and subvalues therebetween, especially including 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 and 95% by weight.

Examples of component (a) are, among others, (meth)acrylates, fumarates and maleates, which derived from saturated alcohols such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl(meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl(meth)acrylate, pentyl(meth)acrylate and hexyl (meth)acrylate; cycloalkyl (meth)acrylates, like cyclopentyl(meth)acrylate.

Furthermore, the monomer compositions to obtain the polymers comprising units derived from alkyl esters contain 0-100 wt %, preferably 10-99 wt %, especially 20-95 wt % and more preferably 30 to 85 wt %, based on the total weight of the monomer mixture, of one or more ethylenically unsaturated ester compounds of formula (II)

where R is hydrogen or methyl, R⁴ means a linear or branched alkyl residue with 7-40, especially 10 to 30 and preferably 12 to 24 carbon atoms, R⁵ and R⁶ are independently hydrogen or a group of the formula —COOR″, where R″ means hydrogen or an alkyl group with 7 to 40, especially 10 to 30 and preferably 12 to 24 carbon atoms. The amount of a compound of formula (II) in the mixture includes all values and subvalues therebetween, especially including 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 and 95% by weight.

Among these are (meth)acrylates, fumarates and maleates that derive from saturated alcohols, such as 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, 2-tert-butylheptyl (meth)acrylate, octyl (meth)acrylate, 3-isopropylheptyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, 2-methylhexadecyl (meth)acrylate, heptadecyl (meth)acrylate, 5-isopropylheptadecyl (meth)acrylate, 4-tert-butyloctadecyl (meth)acrylate, 5-ethyloctadecyl (meth)acrylate, 3-isopropyloctadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, cetyleicosyl (meth)acrylate, stearyleicosyl (meth)acrylate, docosyl (meth)acrylate, and/or eicosyltetratriacontyl (meth)acrylate;

cycloalkyl (meth)acrylates such as 3-vinylcyclohexyl (meth)acrylate, cyclohexyl (meth)acrylate, bornyl (meth)acrylate, 2,4,5-tri-t-butyl-3-vinylcyclohexyl(meth)acrylate, 2,3,4,5-tetra-t-butylcyclohexyl (meth)acrylate; and the corresponding fumarates and maleates.

The ester compounds with a long-chain alcohol residue, especially component (b), can be obtained, for example, by reacting (meth)acrylates, fumarates, maleates and/or the corresponding acids with long chain fatty alcohols, where in general a mixture of esters such as (meth)acrylates with different long chain alcohol residues results. These fatty alcohols include, among others, Oxo Alcohol® 7911 and Oxo Alcohol® 7900, Oxo Alcohol® 1100 (Monsanto); Alphanol® 79 (ICI); Nafol®°1620, Alfol® 610 and Alfol® 810 (Sasol); Epal® 610 and Epal® 810 (Ethyl Corporation); Linevol® 79, Linevol® 911 and Dobanol® 25L (Shell AG); Lial 125 (Sasol); Dehydad® and Dehydad® (and Lorol® (Cognis).

Of the ethylenically unsaturated ester compounds, the (meth)acrylates are particularly preferred over the maleates and furmarates, i.e., R², R³, R⁵, R⁶ of formulas (I) and (II) represent hydrogen in particularly preferred embodiments.

In a particular aspect of the present invention, preference is given to using mixtures of ethylenically unsaturated ester compounds of formula (II), and the mixtures have at least one (meth)acrylate having from 7 to 15 carbon atoms in the alcohol radical and at least one (meth) acrylate having from 16 to 30 carbon atoms in the alcohol radical. The fraction of the (meth)acrylates having from 7 to 15 carbon atoms in the alcohol radical is preferably in the range from 20 to 95% by weight, based on the weight of the monomer composition for the preparation of polymers. The fraction of the (meth)acrylates having from 16 to 30 carbon atoms in the alcohol radical is preferably in the range from 0.5 to 60% by weight based on the weight of the monomer composition for the preparation of the polymers comprising units derived from alkyl esters. The weight ratio of the (meth)acrylate having from 7 to 15 carbon atoms in the alcohol radical and the (meth) acrylate having from 16 to 30 carbon atoms in the alcohol radical is preferably in the range of 10:1 to 1:10, more preferably in the range of 5:1 to 1.5:1.

Component (c) comprises in particular ethylenically unsaturated monomers that can copolymerize with the ethylenically unsaturated ester compounds of formula (I) and/or (II).

Comonomers that correspond to the following formula are especially suitable for polymerization in accordance with the present invention:

where R¹* and R²* independently are selected from the group consisting of hydrogen, halogens, CN, linear or branched alkyl groups with 1-20, preferably 1-6 and especially preferably 1-4 carbon atoms, which can be substituted with 1 to (2n+1) halogen atoms, where n is the number of carbon atoms of the alkyl group (for example CF3), α, β-unsaturated linear or branched alkenyl or alkynyl groups with 2-10, preferably 2-6 and especially preferably 2-4 carbon atoms, which can be substituted with 1 to (2n-1) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the alkyl group, for example CH2═CCl—, cycloalkyl groups with 3-8 carbon atoms, which can be substituted with 1 to (2n-1) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the cycloalkyl group; C(═Y*)R⁵*, C(═Y*)NR⁶*R⁷*, Y*C(═Y*)R⁵*, SOR⁵*, SO₂R⁵*, OSO₂R⁵*, NR⁸*SO₂R⁵*, PR⁵*₂, P(═Y*)R⁵*₂, Y*PR⁵*₂, Y*P(═Y*)R⁵*₂, NR⁸*₂, which can be quaternized with an additional R⁸*, aryl, or heterocyclyl group, where Y* can be NR⁸*, S or O, preferably O; R⁵* is an alkyl group with 1-20 carbon atoms, an alkylthio group with 1-20 carbon atoms, OR¹ (R¹⁵ is hydrogen or an alkali metal), alkoxy with 1-20 carbon atoms, aryloxy or heterocyclyloxy; R⁶* and R⁷* independently are hydrogen or an alkyl group with one to 20 carbon atoms, or R⁶* and R⁷* together can form an alkylene group with 2-7, preferably 2-5 carbon atoms, where they form a 3-8 member, preferably 3-6 member ring, and R⁸* is linear or branched alkyl or aryl groups with 1-20 carbon atoms;

R³* and R⁴* independently are chosen from the group consisting of hydrogen, halogen (preferably fluorine or chlorine), alkyl groups with 1-6 carbon atoms and COOR⁹*, where R⁹* is hydrogen, an alkali metal or an alkyl group with 1-40 carbon atoms, or R¹* and R³* can together form a group of the formula (CH₂)_(n), which can be substituted with 1-2n′ halogen atoms or C₁-C₄ alkyl groups, or can form a group of the formula C(═O)—Y*—C(═O), where n′ is from 2-6, preferably 3 or 4, and Y* is defined as before; and where at least 2 of the residues R¹*, R²*, R³* and R⁴* are hydrogen or halogen.

The comonomers include, among others, hydroxyalkyl(meth)acrylates like 3-hydroxypropyl(meth)acrylate, 3,4-dihydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2,5-dimethyl-1,6-hexanediol (meth)acrylate, 1,10-decanediol (meth)acrylate;

aminoalkyl (meth)acrylates and aminoalkyl (meth)acrylamides like N-(3-dimethylaminopropyl)methacrylamide, 3-diethylaminopentyl (meth)acrylate, 3-dibutylaminohexadecyl (meth)acrylate;

nitriles of (meth)acrylic acid and other nitrogen-containing (meth)acrylates like N-(methacryloyloxyethyl)diisobutylketimine, N-(methacryloyloxyethyl)dihexadecylketimine, (meth)acryloylamidoacetonitrile, 2-methacryloyloxyethylmethylcyanamide, cyanomethyl (meth)acrylate;

aryl (meth)acrylates like benzyl (meth)acrylate or phenyl (meth)acrylate, where the acryl residue in each case can be unsubstituted or substituted up to four times;

carbonyl-containing (meth)acrylates like 2-carboxyethyl (meth)acrylate, carboxymethyl (meth)acrylate, oxazolidinylethyl (meth)acrylate,

N-methyacryloyloxy)formamide, acetonyl (meth)acrylate, N-methacryloylmorpholine, N-methacryloyl-2-pyrrolidinone, N-(2-methyacryloxyoxyethyl)-2-pyrrolidinone, N-(3-methacryloyloxypropyl)-2-pyrrolidinone, N-(2-methyacryloyloxypentadecyl(-2-pyrrolidinone, N-(3-methacryloyloxyheptadecyl-2-pyrrolidinone;

(meth)acrylates of ether alcohols like tetrahydrofurfuryl (meth)acrylate, vinyloxyethoxyethyl (meth)acrylate, methoxyethoxyethyl (meth)acrylate, 1-butoxypropyl (meth)acrylate, 1-methyl-(2-vinyloxy)ethyl (meth)acrylate, cyclohexyloxymethyl (meth)acrylate, methoxymethoxyethyl (meth)acrylate, benzyloxyl methyl (meth)acrylate, furfuryl (meth)acrylate, 2-butoxyethyl (meth)acrylate, 2-ethoxyethoxymethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, ethoxylated (meth)acrylates, allyloxymethyl (meth)acrylate, 1-ethoxybutyl (meth)acrylate, methoxymethyl (meth)acrylate, 1-ethoxyethyl (meth)acrylate, ethoxymethyl (meth)acrylate;

(meth)acrylates of halogenated alcohols like 2,3-dibromopropyl (meth)acrylate, 4-bromophenyl (meth)acrylate, 1,3-dichloro-2-propyl (meth)acrylate, 2-bromoethyl (meth)acrylate, 2-iodoethyl(meth)acrylate, chloromethyl(meth)acrylate;

oxiranyl (meth)acrylate like 2,3-epoxybutyl (meth)acrylate, 3,4-epoxybutyl (meth)acrylate, 10,11 epoxyundecyl (meth)acrylate, 2,3-epoxycyclohexyl (meth)acrylate, oxiranyl (meth)acrylates such as 10,11-epoxyhexadecyl(meth)acrylate, glycidyl (meth)acrylate;

phosphorus-, boron- and/or silicon-containing (meth)acrylates like 2-(dimethylphosphato)propyl (meth)acrylate, 2-(ethylphosphito) propyl (meth)acrylate, 2-dimethylphosphinomethyl (meth)acrylate, dimethylphosphonoethyl (meth)acrylate, diethylmethacryloyl phosphonate, dipropylmethacryloyl phosphate, 2 (dibutylphosphono)ethyl(meth)acrylate, 2,3-butylenemethacryloylethyl borate, methyldiethoxymethacryloylethoxysiliane, diethylphosphatoethyl (meth)acrylate;

sulfur-containing (meth)acrylates like ethylsulfinylethyl(meth)acrylate, 4-thiocyanatobutyl(meth)acrylate, ethylsulfonylethyl(meth)acrylate, thiocyanatomethyl (meth)acrylate, methylsulfinylmethyl (meth)acrylate, bis(methacryloyloxyethyl) sulfide;

heterocyclic (meth)acrylates like 2-(1-imidazolyl)ethyl(meth)acrylate, 2-(4-morpholinyl)ethyl(meth)acrylate and 1-(2-methacryloyloxyethyl)-2-pyrrolidone;

vinyl halides such as, for example, vinyl chloride, vinyl fluoride, vinylidene chloride and vinylidene fluoride;

vinyl esters like vinyl acetate;

vinyl monomers containing aromatic groups like styrene, substituted styrenes with an alkyl substituent in the side chain, such as α-methylstyrene and α-ethylstyrene, substituted styrenes with an alkyl substituent on the ring such as vinyltoluene and α-methylstyrene, halogenated styrenes such as monochlorostyrenes, dichlorostyrenes, tribromostyrenes and tetrabromostyrenes;

heterocyclic vinyl compounds like 2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine, vinylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole, 2-methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactam, N-vinylbutyrolactam, vinyloxolane, vinylfuran, vinylthiophene, vinylthiolane, vinylthiazoles and hydrogenated vinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles;

vinyl and isoprenyl ethers;

maleic acid derivatives such as maleic anhydride, methylmaleic anhydride, maleinimide, methylmaleinimide;

fumaric acid and fumaric acid derivatives such as, for example, mono- and diesters of fumaric acid.

Monomers that have dispersing functionality can also be used as comonomers. These monomers contain usually hetero atoms such as oxygen and/or nitrogen. For example the previously mentioned hydroxyalkyl(meth)acrylates, aminoalkyl (meth)acrylates and aminoalkyl (meth)acrylamides, (meth)acrylates of ether alcohols, heterocyclic (meth)acrylates and heterocyclic vinyl compounds are considered as dispersing comononers.

Especially preferred mixtures contain methyl methacrylate, lauryl methacrylate and/or stearyl methacrylate.

The components can be used individually or as mixtures.

The hydraulic fluid of the present invention preferably comprises polyalkylmethacrylate polymers. These polymers are obtainable by polymerizing compositions comprising alkyl-methacrylate monomers. Preferably, these polyalkylmethacrylate polymers comprise at least 40% by weight, especially at least 50% by weight, more preferably at least 60% by weight and most preferably at least 80% by weight methacrylate repeating units. Preferably, these polyalkylmethacrylate polymers comprise C₉-C₂₄ methacrylate repeating units and C₁-C₈ methacrylate repeating units.

The molecular weight of the polymers derived from alkyl esters is not critical. Usually the polymers derived from alkyl esters have a molecular weight in the range of 300 to 1,000,000 g/mol, preferably in the range of range of 10000 to 200,000 g/mol and more preferably in the range of 25000 to 100,000 g/mol, without any limitation intended by this. These values refer to the weight average molecular weight of the polymers.

Without intending any limitation by this, the alkyl(meth)acrylate polymers exhibit a polydispersity, given by the ratio of the weight average molecular weight to the number average molecular weight Mw/Mn, in the range of 1 to 15, preferably 1.1 to 10, especially preferably 1.2 to 5. The polydispersity may be determined by gel permeation chromatography (GPC). The preferred polydipersity includes all values and subvalues therebetween, especially including 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.5, 3, 3.5, 4 and 4.5.

The monomer mixtures described above can be polymerized by any known method. Conventional radical initiators can be used to perform a classic radical polymerization. Examples for these radical initiators are azo initiators like 2,2′-azodiisobutyronitrile (AIBN), 2,2′-azobis(2-methylbutyronitrile) and 1,1 azo-biscyclohexane carbonitrile; peroxide compounds, e.g. methyl ethyl ketone peroxide, acetyl acetone peroxide, dilauryl peroxide, tert.-butyl per-2-ethyl hexanoate, ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert.-butyl perbenzoate, tert.-butyl peroxy isopropyl carbonate, 2,5-bis(2-ethylhexanoyl-peroxy)-2,5-dimethyl hexane, tert.-butyl peroxy 2-ethyl hexanoate, tert.-butyl peroxy-3,5,5-trimethyl hexanoate, dicumene peroxide, 1,1 bis(tert. butyl peroxy)cyclohexane, 1,1 bis(tert. butyl peroxy) 3,3,5-trimethyl cyclohexane, cumene hydroperoxide and tert.-butyl hydroperoxide.

Low weight average molecular weight poly(meth)acrylates can be obtained by using chain transfer agents. This technology is ubiquitously known and practiced in the polymer industry and is described in Odian, Principles of Polymerization, 1991. Examples of chain transfer agents are sulfur containing compounds such as thiols, e.g. n- and t-dodecanethiol, 2-mercaptoethanol, and mercapto carboxylic acid esters, e.g. methyl-3-mercaptopropionate Preferred chain transfer agents contain up to 20, especially up to 15 and more preferably up to 12 carbon atoms. Furthermore, chain transfer agents may contain at least 1, especially at least 2 oxygen atoms.

Furthermore, the low weight average molecular weight poly(meth)acrylates can be obtained by using transition metal complexes, such as low spin cobalt complexes. These technologies are well known and for example described in USSR patent 940,487-A and by Heuts, et al., Macro-molecules 1999, pp 2511-2519 and 3907-3912.

Furthermore, polymerization techniques such as ATRP (Atom Transfer Radical Polymerization) and or RAFT (Reversible Addition Fragmentation Chain Transfer) can be applied to obtain useful polymers derived from alkyl esters. The ATRP reaction method is described, for example, by J- S. Wang, et al., J. Am. Chem. Soc., Vol. 117, pp. 5614-5615 (1995), and by Matyjaszewski, Macromolecules, Vol. 28, pp. 7901-7910 (1995). Moreover, the patent applications WO 96/30421, WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclose variations of the ATRP explained above to which reference is expressly made for purposes of the disclosure. The RAFT method is extensively presented in WO 98/01478, for example, to which reference is expressly made for purposes of the disclosure.

The polymerization can be carried out at normal pressure, reduced pressure or elevated pressure. The polymerization temperature is also not critical. However, in general it lies in the range of −20-200° C., preferably 0-130° C. and especially preferably 60-120° C., without any limitation intended by this.

The polymerization can be carried out with or without solvents. The term solvent is to be broadly understood here.

According to a preferred embodiment, the polymer is obtainable by a polymerization in API Group II or Group III mineral oil. These solvents are disclosed above.

Furthermore, polymers obtainable by polymerization in a polyalphaolefin (PAO) are preferred. More preferably, the PAO has a number average molecular weight in the range of 200 to 10000, more preferably 500 to 5000 g/mol. This solvent is disclosed above.

The hydraulic fluid may comprise 0.5 to 50% by weight, especially 1 to 30% by weight, and preferably 3 to 20% by weight, based on the total weight of the fluid, of one or more polymers derived from alkyl esters. According to a preferred embodiment of the present invention, the hydraulic fluid comprises at least 5% by weight of one or more polymers derived from alkyl esters. The amount of one or more polymers derived from alkyl esters includes all values and subvalues therebetween, especially including 5, 10, 15, 20, 25, 30, 35, 40 and 45% by weight.

According to a preferred aspect of the present invention, the fluid may comprise at least two polymers having a different monomer composition. Preferably at least one of the polymers is derived from alkyl esters. In another embodiment, at least one of the polymers is a polyolefin. A preferred combination is the use of a polymer derived from an alkyl ester, and a polymer derived from polyolefins. Preferably, the polyolefin is useful as a viscosity index improver.

These polyolefins include in particular polyolefin copolymers (OCP) and hydrogenated styrene/diene copolymers (HSD). The polyolefin copolymers (OCP) to be used according to the present invention are primarily polymers synthesized from ethylene, propylene, isoprene, butylene and/or further olefins having 5 to 20 carbon atoms. Systems which have been grafted with small amounts of oxygen- or nitrogen-containing monomers (e.g. from 0.05 to 5% by weight of maleic anhydride) may also be used. The copolymers which contain diene components are generally hydrogenated in order to reduce the oxidation sensitivity and the crosslinking tendency of the viscosity index improvers.

The weight average molecular weight Mw is in general from 10 000 to 300 000, preferably between 50 000 and 150 000. Such olefin copolymers are described, for example, in the German Laid-Open Applications DE-A 16 44 941, DE-A 17 69 834, DE-A 19 39 037, DE-A 19 63 039, and DEA 20 59 981.

Ethylene/propylene copolymers are particularly useful and terpolymers having ternary components, such as ethylidene-norbornene (cf. Macromolecular Reviews, Vol. 10 (1975)) are also possible, but their tendency to crosslink must also be taken into account in the aging process. The distribution may be substantially random, but sequential polymers comprising ethylene blocks can also advantageously be used. The ratio of the monomers ethylene/propylene is variable within certain limits, which can be set to about 75% for ethylene and about 80% for propylene as an upper limit. Owing to its reduced tendency to dissolve in oil, polypropylene is less suitable than ethylene/propylene copolymers. In addition to polymers having a predominantly atactic propylene incorporation, those having a more pronounced isotactic or syndiotactic propylene incorporation may also be used.

Such products are commercially available, for example under the trade names Dutral® CO 034, Dutral® CO 038, Dutral® CO 043, Dutral® CO 058, Buna® EPG 2050 or Buna® EPG 5050.

The hydrogenated styrene/diene copolymers (HSD) are being described, for example, in DE 21 56 122. They are in general hydrogenated isoprene/styrene or butadiene/styrene copolymers. The ratio of diene to styrene is preferably in the range from 2:1 to 1:2, particularly preferably about 55:45. The weight average molecular weight Mw is in general from 10000 to 300 000, preferably between 50000 and 150000 g/mol. According to a particular aspect of the present invention, the proportion of double bonds after the hydrogenation is not more than 15%, particularly preferably not more than 5%, based on the number of double bonds before the hydrogenation.

Hydrogenated styrene/diene copolymers can be commercially obtained under the trade name SHELLVIS® 50, 150, 200, 250 or 260.

Preferably, at least one of the polymers of the mixture comprises units derived from monomers selected from acrylate monomers, methacrylate monomers, fumarate monomers and/or maleate monomers. These polymers are described above.

The weight ratio of the polyolefin and the polymer comprises units derived from monomers selected from acrylate monomers, methacrylate monomers, fumarate monomers and/or maleate monomers may be in the range of 1:10 to 10:1, especially 1:5 to 5:1.

The hydraulic fluid may comprise usual additives. These additive include e.g. antioxidants, antiwear agents, corrosion inhibitors and/or defoamers, often purchased as a commercial additive package.

Preferably, the hydraulic fluid has a viscosity according to ASTM D 445 at 40° C. in the range of 10 to 150 mm²/s, more preferably 22 to 100 mm²/s.

Preferably, the hydraulic system includes the following components:

-   -   1. A unit creating mechanical energy, e.g. a combustion engine         or an electrical motor.     -   2. A fluid flow or force-generating unit that converts         mechanical energy into hydraulic power, such as a pump.     -   3. Piping for transmitting fluid under pressure.     -   4. A unit that converts the hydraulic power of the fluid into         mechanical work or motion, such as an actuator or fluid motor.         There are two types of motors, cylindrical and rotary.     -   5. A control circuit with valves that regulate flow, pressure,         direction of movement, and applied forces.     -   6. A fluid reservoir that allows for separation of water, foam,         entrained air, or debris before the clean fluid is returned to         the system through a filter.     -   7. A liquid with low compressibility capable of operating         without degradation under the conditions of the application         (temperature, pressure, radiation).

Most complex systems will make use of multiple pumps, rotary motors, cylinders, electronically controlled with valves and regulators.

According to a preferred embodiment of the present invention, a vane pump or a piston pump can be used in order to create hydraulic power.

The system may be operated at high pressures. The improvement of the present invention can be achieved at pressures in the range of 50 to 700 bars, preferably 100 to 400 bars and more preferably 150 to 350 bars.

The unit creating mechanical energy, e.g. a motor can be operated at a speed of 500 to 5000 rpm, preferably 1000 to 3000 rpm and more preferably 1400 to 2000 rpm.

The hydraulic fluid can be used over a wide temperature window. Preferably, the fluid can be used at a temperature in the range of −30° C. to 200° C., more preferably 10° C. to 150° C., even more preferably 20-00° C., and most preferably 20-50° C. Usually, the operating temperature depends on the base fluid used to manufacture the hydraulic fluid.

Preferably, the fluid is used in military hydraulic systems, in hydraulic launch assist systems for hybrid propulsion vehicles, in industrial, marine, mining and/or mobile equipment hydraulic systems.

Furthermore, the present invention provides a hydraulic system comprising a hydraulic fluid having a VI of at least 130, a unit for creating mechanical power, a unit that converts mechanical power into hydraulic power, and a unit that converts hydraulic power into mechanical work or motion.

Preferentially, engine speed can be maintained at a constant level to deliver higher amounts of hydraulic power. Preferably, the mechanical power output of the engine or electrical motor can be operated at its full power capacity to deliver higher amounts of hydraulic power compared to the hydraulic system utilizing a standard HM grade fluid with a viscosity index less than 120.

Moreover, the increase in power output from a hydraulic system can be magnified when a pump and rotary motor are combined. In such a system, the use of a hydraulic fluid with high viscosity index according to the present invention will increase the volume flow rate and power output of the pump, which then flows to one or more rotary motors. This effect is repeated in the rotary motor and thus multiplies the increase in total power output from the system. As a simple example, a high viscosity index fluid is substituted for an HM fluid to increase the power output of a pump by 5%. That fluid then flows into a rotary motor which also experiences a 5% increase in power output as it provides mechanical work. The total power output of the pump and motor system is thus improved by over 10%, as described in the following simple example.

System A with HM oil-10.0 kW into the pump from the engine, 9.0 kW out of the pump into the rotary motor, 8.1 kW out of the motor as mechanical work.

System B with HVI oil-10.0 kW into the pump from the engine, 9.45 kW out of the pump into the rotary motor, 8.93 kW out of the motor as mechanical work.

The hydraulic pump from System B delivers 5% more power than the pump from System A [(9.45-9.0)/9.0=0.05×100=5%].

The hydraulic system System B delivers 10.2% more power than System A [(8.93-8.1)/8.1=0.102×100=10.2%].

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.

EXAMPLES Examples 1 and 2 and Comparative Example 1

A Denison T6C mobile vane pump was operated under the following controlled conditions:

Speed=1500 rpm, Pressure=200 bars, Fluid Temperature=80° C. An ISO VG 46 HM oil was run as a reference fluid, generated 6.97 kW of hydraulic power.

By comparison, several ISO VG 46 HV oils were run under the same conditions, and generated 6 to 11% higher levels of hydraulic power output, as shown in Table 1.

TABLE 1 Denison T6C Mobile Vane Pump Power Output Comparative Example 1 Example 1 Example 2 Fluid Type ISO 46 HM ISO 46 HV ISO 46 HV Viscosity Index 100 150 200 Hydraulic 6.97 7.42 7.74 Power Output, kW Power Output 6.6% 11.1% Increase, % Operating Conditions: 1500 rpm, 200 bars, 80° C.

Examples 3 to 5 and Comparative Example 2

An Eaton-Vickers V20 vane pump was operated under the following controlled conditions: Speed=1200 rpm, Pressure=138 bars, Fluid Temperature=80° C.

An ISO VG 46 HM oil was run as a reference fluid, generated 8.69 kW of hydraulic power.

By comparison, several ISO VG 46 HV oils were run under the same conditions, and generated 3 to 6% higher levels of hydraulic power output, as shown in Table 2.

TABLE 2 Eaton-Vickers V20 Vane Pump Power Output Comparative Example 2 Example 3 Example 4 Example 5 Fluid Type ISO 46 HM ISO 46 HV ISO 46 HV ISO 46 HV Viscosity Index 100 160 180 200 Hydraulic 8.69 9.01 9.09 9.16 Power Output, kW Power Output 3.8% 4.6% 5.5% Increase, % Operating Conditions: 1200 rpm, 138 bars, 80° C.

Examples 6 to 8 and Comparative Example 3

An Eaton-Vickers V104C vane pump was operated under the following controlled conditions:

Speed=1200 rpm, Pressure=138 bars, Fluid Temperature=80° C. An ISO VG 46 HM oil was run as a reference fluid, generated 8.35 kW of hydraulic power.

By comparison, several ISO VG 46 HV oils were run under the same conditions, and generated 5 to 7% higher levels of hydraulic power output, as shown in Table 3.

TABLE 3 Eaton-Vickers V104C Vane Pump Power Output Comparative Example 3 Example 6 Example 7 Example 8 Fluid Type ISO 46 HM ISO 46 HV ISO 46 HV ISO 46 HV Viscosity Index 100 160 180 200 Hydraulic 8.35 8.76 8.86 8.95 Power Output, kW Power Output 4.9% 6.0% 7.2% Increase, % Operating Conditions: 1200 rpm, 138 bars, 80° C.

Examples 9 to 11 and Comparative Example 4

A Komatsu 35+35 dual piston pump was operated under the following controlled conditions:

Speed=2100 rpm, Pressure=350 bars, Fluid Temperature=100° C. An ISO VG 46 HM oil was run as a reference fluid, generated 5.83 kW of hydraulic power.

By comparison, several ISO VG 46 HV oils were run under the same conditions, and generated 4 to 6% higher levels of hydraulic power output, as shown in Table 4.

TABLE 4 Komatsu 35 + 35 Dual Piston Pump Power Output Comparative Example 4 Example 9 Example 10 Example 11 Fluid Type ISO 46 HM ISO 46 HV ISO 46 HV ISO 46 HV Viscosity Index 100 160 180 200 Hydraulic 5.83 6.07 6.13 6.18 Power Output, kW Power Output 4.0% 5.0% 6.0% Increase, % Operating Conditions: 2100 rpm, 350 bars, 100° C.

Examples 12 to 14 and Comparative Example 5

An Eaton L2 gear pump was operated under the following controlled conditions:

Speed=2750 rpm, Pressure=207 bars, Fluid Temperature=80° C. An ISO VG 46 HM oil was run as a reference fluid, generated 21.5 kW of hydraulic power.

By comparison, several ISO VG 46 HV oils were run under the same conditions, and generated 6 to 8.8% higher levels of hydraulic power output, as shown in Table 5. The pump flow rate and the power output data were provided by the manufacturer and the increase in power output was calculated compared to Comparative Example 5.

TABLE 5 Eaton L2 Gear Pump Power Output Comparative Example 5 Example 12 Example 13 Example 14 Fluid Type ISO 46 HM ISO 46 HV ISO 46 HV ISO 46 HV Viscosity Index 100 160 180 200 Hydraulic 21.5 22.8 23.2 23.4 Power Output, kW Power Output 6.0% 7.5% 8.8% Increase, % Operating Conditions: 2750 rpm, 207 bars, 80° C.

The data gathered in the examples demonstrate that the “HV” multigrade oil formulated with Group II PAMA was responsible for increased hydraulic power output from the hydraulic pumps. The increased work output allowed the excavator to complete the work cycle in a shorter period of time, and thus complete higher levels of work output in equivalent periods of time.

Examples 15 to 18 and Comparative Example 6

A further advantage of the present invention is to design a hydraulic system that operates at a lower pressure level and delivers an equivalent amount of hydraulic power output. Table 6 contains data comparing the relative power input and hydraulic power output in a Denison P09 piston pump at about 5000 psi (345 bar). The pump flow rate and the power output data were provided by the manufacturer and the increase in power output was calculated compared to Comparative Example 6.

TABLE 6 Comparison of Piston Pump Power Output at Lower Pressure % Change % Change Flow Rate, Power Overall Power Overall Pressure, psi gpm In, kW Efficiency In Efficiency Comparative 5000, 46.76 156.2 79.5 Example 6 Monograde Example 15 5000 psi, 48.87 149.9 82.9 −4.1 4.3 MEHF Example 16 4950 psi, 48.96 148.2 83 −5.2 4.4 MEHF Example 17 4750 psi, 49.31 141.2 83.5 −9.6 5.0 MEHF Example 18 4500 psi, 49.75 132.8 84.2 −15.0 5.9 MEHF Hydraulic % Change Power % Change in power power (kW) power out out/power in out/power in Comparative 101.72 0.651 Example 6 Example 15 106.31 4.5 0.709 8.9 Example 16 105.44 3.7 0.712 9.3 Example 17 101.90 0.2 0.722 10.8 Example 18 97.40 −4.2 0.733 12.6

The data was also expressed graphically in FIG. 1, and demonstrated that a system can be designed to operate at a 5% lower pressure level which delivers an equivalent level of hydraulic power output.

Examples 19 to 22 and Comparative Example 7

A further experiment showed the improvement of the relative power input and hydraulic power output in a Denison T6C vane pump at about 3000 psi. (207 bar) being achieved by the present invention. The results achieved are shown in Table 7. Additionally, the data is also expressed graphically in FIG. 2, and demonstrated that a system can be designed to operate at a 6% lower pressure level which delivers an equivalent level of hydraulic power output.

TABLE 7 Comparison of Vane Pump Power Output at Lower Pressure % Change % Change Flow Rate, Power Overall Power Overall Pressure, psi lpm In, kW Efficiency In Efficiency Comparative 3000, 50.45 24.86 70.01 Example 7 Monograde Example 19 3000 psi, 52.56 24.95 72.68 0.4 3.8 MEHF Example 20 2970 psi, 52.72 24.71 72.88 −0.6 4.1 MEHF Example 21 2850 psi, 53.32 23.78 73.61 −4.3 5.1 MEHF Example 22 2700 psi, 54.15 22.49 74.63 −9.5 6.6 MEHF % Change Hydraulic Power % Change in power power (kW) power out out/power in out/power in Comparative 16.82 0.676 Example 7 Example 19 17.52 4.2 0.702 3.8 Example 20 17.40 3.5 0.704 4.1 Example 21 16.88 0.4 0.710 5.0 Example 22 16.25 −3.4 0.722 6.8 

1. A method of improving a power output of a hydraulic system, comprising: operating said hydraulic system with a hydraulic fluid having a VI of at least
 130. 2. The method according to claim 1, wherein said power output is increased at least 3% compared to the power output of a hydraulic system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor.
 3. The method according to claim 1, wherein said power output is increased at least 5% compared to the power output of a hydraulic system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor.
 4. A method of improving a volume output of a hydraulic system, comprising: operating said hydraulic system with a hydraulic fluid having a VI of at least 130; wherein the volume output of said hydraulic system is increased compared to the volume output of a system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor.
 5. The method according to claim 4, wherein the volume output is increased at least 3%.
 6. The method according to claim 4, wherein the volume output is increased at least 5%.
 7. The method according to claim 1, wherein a constancy of the power output is increased.
 8. The method according to claim 1, wherein a constancy of the power output is increased at the maximum load.
 9. The method according to claim 7, wherein the drop of the power output after at least 10 minutes of operating time is at most 3%, measured at a load of 90% of the maximum load or more of a unit providing mechanical energy.
 10. The method according to claim 1, wherein the engine speed of a unit providing mechanical energy is maintained at a constant rate and the system delivers an increased level of hydraulic power.
 11. The method according to claim 1, wherein the pressure provided by a unit providing hydraulic power is in the range of 50 to 700 bar.
 12. The method according to claim 1, wherein the pressure provided by a unit providing hydraulic power is in the range of 150 to 350 bar.
 13. The method according to claim 1, wherein the hydraulic system is designed to operate at a lower pressure, such that the output power is equivalent to that delivered by a reference system using a hydraulic fluid with a VI of
 100. 14. The method according to claim 1, wherein the hydraulic system demonstrates an improvement in the ratio of hydraulic power output to power input, such that the ratio of power output/power input is improved by at least 3%, compared to that delivered by a reference system using a hydraulic fluid with a VI of
 100. 15. The method according to claims 1, wherein the hydraulic fluid has a VI of at least
 150. 16. The method according to claim 1, wherein the hydraulic fluid has a VI of at least
 180. 17. The method according to claim 1, wherein the hydraulic fluid is a NFPA double viscosity grade, triple viscosity grade, quadra viscosity grade, or penta viscosity grade hydraulic fluid.
 18. The method according to claim 1, wherein the hydraulic fluid is obtained by mixing a base fluid and a polymeric viscosity index improver.
 19. The method according to claim 1, wherein the hydraulic fluid comprises at least 60% by weight of at least one base fluid.
 20. The method according to claim 1, wherein the hydraulic fluid comprises at least 60% by weight of at least one base fluid having a viscosity index of 120 or less.
 21. The method according to claim 1, wherein the hydraulic fluid comprises a member selected from the group consisting of a mineral oil, a synthetic oil and mixtures thereof.
 22. The method according to claim 1, wherein the hydraulic fluid comprises an API group I oil, API group II oil, API group III oil, a API group IV oil, API group V oil, a Fischer-Tropsch (GTL) derived oil or mixtures thereof.
 23. The method according to claim 1, wherein the hydraulic fluid comprises a polyalphaolefin, a carboxylic ester, a vegetable ester, a phosphate ester, a polyalkylene glycol or mixtures thereof.
 24. The method according to claim 1, wherein the hydraulic fluid comprises at least one polymer.
 25. The method according to claim 24, wherein the polymer comprises polymerized units from monomers selected from the group consisting of acrylate monomers, methacrylate monomers, fumarate monomers, maleate monomers and mixtures thereof.
 26. The method according to claim 1, wherein the hydraulic fluid comprises a polyalkylmethacrylate polymer.
 27. The method according to claim 1, wherein the hydraulic fluid comprises a polymer obtained by polymerizing a mixture of olefinically unsaturated monomers, which comprises a) 0-100 wt % of one or more ethylenically unsaturated ester compounds of formula (I) based on the total weight of the ethylenically unsaturated monomers:

wherein R is hydrogen or methyl, R¹ is a linear or branched alkyl residue with 1-6 carbon atoms, R² and R each independently represent hydrogen or a group of the formula COO′, wherein R′ is hydrogen or an alkyl group with 1-6 carbon atoms, b) 0-100 wt % of one or more ethylenically unsaturated ester compounds of formula (II) based on the total weight of the ethylenically unsaturated monomers:

wherein R is hydrogen or methyl, R⁴ is a linear or branched alkyl residue with 7-40 carbon atoms, R⁵ and R⁶ independently are hydrogen or a group of the formula —COOR′, wherein R″ is hydrogen or an alkyl group with 7-40 carbon atoms, c) 0-50 wt % of comonomers based on the total weight of the ethylenically unsaturated monomers.
 28. The method according to claim 24, wherein the polymer is obtained by a polymerization in a API group II mineral oil or API group III mineral oil.
 29. The method according to claim 24, wherein the polymer is obtained by a polymerization in a polyalphaolefin.
 30. The method according to claim 24, wherein the polymer is obtained by polymerizing a dispersant monomer.
 31. The method according to claim 24, wherein the polymer is obtained by polymerizing a vinyl monomer containing an aromatic group.
 32. The method according to claim 24, wherein the polymer has a weight average molecular weight in the range of 10000 to 200000 g/mol.
 33. The method according to claim 1, wherein the hydraulic fluid comprises 0.5 to 40% by weight of a polymer.
 34. The method according to claim 1, wherein the hydraulic fluid comprises 3 to 20% by weight of a polymer.
 35. The method according to claim 24, wherein the hydraulic fluid comprises at least two polymers having a different monomer composition.
 36. The method according to claim 35, wherein at least one of the polymers is a polyolefin.
 37. The method according to claim 36, wherein at least one of the polymers comprises units derived from at least one alkyl ester monomer.
 38. The method according to claim 37, wherein a weight ratio of the polyolefin and the polymer comprising units derived from at least one alkyl ester monomer is in the range of 1:10 to 10:1.
 39. The method according to claim 1, wherein the hydraulic fluid comprises an oxygen containing compound selected from the group consisting of carboxylic acid esters, polyether polyols, organophosphorus compounds and mixtures thereof.
 40. The method according to claim 39, wherein the oxygen containing compound is a carboxylic ester containing at least two ester groups.
 41. The method according to claim 39, wherein the oxygen containing compound is a diester of a carboxylic acid containing 4 to 12 carbon atoms.
 42. The method according to claim 39, wherein the oxygen containing compound is an ester of a polyol.
 43. The method according to claim 1, wherein the hydraulic fluid has an ISO viscosity grade in the range of 15 to
 150. 44. The method according to claim 1, wherein the hydraulic fluid is used at a temperature in the range of −40° C. to 120° C.
 45. The method according to claim 1, wherein the hydraulic fluid comprises a member selected from the group consisting of antioxidants, antiwear agents, corrosion inhibitors, defoamers and mixtures thereof.
 46. The method according to claim 1, wherein said hydraulic system is a military hydraulic system, a hydraulic launch assist system for hydraulic hybrid vehicle propulsion, an industrial hydraulic system, marine hydraulic system, mining hydraulic system, mobile equipment hydraulic system or combinations thereof.
 47. The method according to claim 1, wherein said hydraulic system comprises at least one unit providing mechanical energy, at least one unit that converts mechanical energy into hydraulic power, at least one pipe for transmitting hydraulic fluid under pressure and at least a unit that converts the hydraulic power of the hydraulic fluid into mechanical work.
 48. The method according to claim 47, wherein the unit providing mechanical energy comprises a combustion engine.
 49. The method according to claim 47, wherein the unit converting mechanical energy into hydraulic power is a vane pump.
 50. The method according to claim 47, wherein the unit converting mechanical energy into hydraulic power is a piston pump.
 51. The method according to claim 47, wherein the unit converting mechanical energy into hydraulic power is a gear pump.
 52. A hydraulic system, comprising: a hydraulic fluid having a VI of at least 130; wherein a power output of said hydraulic system is increased at least 3% compared to the power output of a hydraulic system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor; wherein said hydraulic system is a military hydraulic system, a hydraulic launch assist system for hydraulic hybrid vehicle propulsion, an industrial hydraulic system, marine hydraulic system, mining hydraulic system, mobile equipment hydraulic system or combinations thereof.
 53. A hydraulic system, comprising: a hydraulic fluid having a VI of at least 130; wherein a power output of said hydraulic system is increased at least 3% compared to the power output of a hydraulic system using a monograde hydraulic fluid having a VI of about 100, operating at the same pressure and temperature with identical mechanical power input from the engine or electric motor; wherein said hydraulic system comprises at least one unit providing mechanical energy, at least one unit that converts mechanical energy into hydraulic power, at least one pipe for transmitting hydraulic fluid under pressure and at least a unit that converts the hydraulic power of the hydraulic fluid into mechanical work.
 54. The hydraulic system according to claim 53, wherein the unit providing mechanical energy comprises a combustion engine.
 55. The hydraulic system according to claim 53, wherein the unit converting mechanical energy into hydraulic power is a vane pump.
 56. The hydraulic system according to claim 53, wherein the unit converting mechanical energy into hydraulic power is a piston pump.
 57. The hydraulic system according to claim 53, wherein the unit converting mechanical energy into hydraulic power is a gear pump. 